This reminds me of “One Tamriel” in TESO, Lore wise and from PvP perspective the worst decision ever, the fact you can do dungeons with enemies and right after kill your group mates in Cyrodil or BGs is the worst MMORPG concept ever.
However, in WoW it’s less dramatic and not the end of the world, actually it only improved freedom of choice within your faction and made raids more balanced. (Shaman windfury, Pala buffs).